1. Field of the Invention
The present invention is directed to methods of applying clear coats on the exterior of substrates. More specifically, the invention relates to methods of applying coatings to the exterior of golf balls.
2. Description of the Related Art
Clear coats are used on the exterior surfaces of a variety of substrates. Clear coats serve to enhance the aesthetic appearance of the substrate as well as act as a barrier to protect the substrate from weathering, mechanical agitation and the like. One such substrate in which clear coats are of particular importance is a golf ball.
A golf ball generally comprises a one-piece construction or it may include several layers including an outer cover surrounding the core. Typically, one or more layers of paint and/or clear coat are applied to the outer surface of the golf ball. For example, in one typical design, the outer surface of the golf ball is first painted with at least one clear or pigmented basecoat primer along with at least one application of a clear top coat. The basecoat and/or primer and clear top coat are applied to the golf ball to enhance the aesthetic appearance of the ball as well as mask or cover surface blemishes that may have resulted from the manufacturing process. The clear top coat is particularly important, in that the coat protects any markings, trademarks, logos, or the like that may be placed on the cover of the ball.
One typical top coat for golf balls utilizes a solvent borne two-component polyurethane that is applied to the exterior of a golf ball. Unfortunately, such top coat formulations require the use of a solvent that is a significant source of volatile organic compounds (VOC's). The presence of VOC's within the solvent component poses numerous environmental and health problems. Consequently, there is thus a need to develop a top coat that is substantially free of volatile compounds.
Attempts have been made to develop coatings for use with substrates other than golf balls that are substantially if not entirely solvent free. One particular area of interest is ultra violet (UV) curable top or clear coats. These top coats utilize radiation in the ultraviolet spectrum to cure a top coat on the surface of a golf ball without the use of solvents thereby eliminating or substantially reducing the presence of VOC's. The UV radiation is used to trigger a polymerization reaction and cure the top coat.
U.S. Pat. No. 5,453,451 ( "the '451 patent") to Sokol discloses a finishing composition which is curable by UV radiation that is substantially free of solvents. The finishing composition of the '451 patent includes a polymerizable compound which is an acrylate and a photoinitiator. The polymerizable compound is from about 80 to about 99.5 percent of the total weight of the composition. The photoinitiator is from about 0.5 percent to about 15 percent of the total weight of the composition. Instead of solvents, a low molecular weight mono or di-acrylate monomer is added to the composition of the 451 patent to control the viscosity for spraying it onto a substrate. Unfortunately, the presence of low molecular weight dilutents as recited in the '451 patent is disadvantageous in that the low molecular weight dilutents increase the toxicity of the composition.
In addition, previous UV-curable coating formulations have generally been at a disadvantage for use as a coating for golf balls to the more traditional two-component polyurethane top coats. For example, previous UV-curable coatings have had deficiencies in abrasion resistance, which is a measure of the ability of the coating to retain and maintain its glossiness in response to weathering and use. These coatings have also had a poor resistance to dirt-pick-up, a related measure of the abrasion resistance of a coating.
Moreover, such coatings have generally poor adhesion qualities in primer/basecoat applications in addition to "direct-to-cover" methods. Adhesion as used herein is the ease to which the top coat bonds to the cover and is required to protect any trademark, lettering, logo, or the like that is on the golf ball surface. In addition, previous UV-curable coatings had a tendency to discolor more rapidly and to a greater extent than the two-component polyurethane coatings. Moreover, an aesthetically unappealing yellowing of the coating is often present in current UV formulations.
These deficiencies have been traced to a variety of factors inherent in traditional UV-curable compositions. For example, UV-curable compositions are preferably cured in air. The oxygen present in the air will interfere with the transmission of UV energy to the reactants. The oxygen will also be transformed into ozone upon absorption of sufficient UV energy. Additionally, the oxygen reacts with the reactants, especially the photoiniatior, thereby necessitating greater amounts of the reactants to form the composition.
A related factor is the need for a higher intensity UV lamp, or operating an UV lamp at a higher intensity to compensate for the oxygen or reactant deficiencies. The higher intensity UV lamp will add to the yellowing of the coating as well as consuming a greater amount of energy. Another factor that adds to yellowing is the choice of photoiniators for the composition. Yellowing may also be caused by the use of particular low molecular weight monomers. These low molecular weight monomers also have toxicity problems and cause brittleness in the UV curable composition. Usually, higher molecular weight oligomers must be added to the composition to compensate for this brittleness.
Consequently, there remains a need for a UV-curable coating that retains the beneficial aspects of a traditional two-component polyurethane formulation without the negative side-effects that are present in current UV-curable formulations. Moreover, there remains a need for a method of curing the UV-curable coating without forming the characteristic yellowing that is prevalent in most UV-curable compositions. Such a coating would be able to be applied using traditional types of spray equipment and contain little or no VOC's.